common/collect/StandardTable.java

/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Predicates.alwaysTrue;
import static com.google.common.base.Predicates.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.not;
import static com.google.common.collect.Maps.safeContainsKey;
import static com.google.common.collect.Maps.safeGet;
import static com.google.common.collect.NullnessCasts.uncheckedCastNullableTToT;
import static java.util.Objects.requireNonNull;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Supplier;
import com.google.common.collect.Maps.IteratorBasedAbstractMap;
import com.google.common.collect.Maps.ViewCachingAbstractMap;
import com.google.common.collect.Sets.ImprovedAbstractSet;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.WeakOuter;
import java.io.Serializable;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import javax.annotation.CheckForNull;

/**
 * {@link Table} implementation backed by a map that associates row keys with column key / value
 * secondary maps. This class provides rapid access to records by the row key alone or by both keys,
 * but not by just the column key.
 *
 * <p>The views returned by {@link #column}, {@link #columnKeySet()}, and {@link #columnMap()} have
 * iterators that don't support {@code remove()}. Otherwise, all optional operations are supported.
 * Null row keys, columns keys, and values are not supported.
 *
 * <p>Lookups by row key are often faster than lookups by column key, because the data is stored in
 * a {@code Map<R, Map<C, V>>}. A method call like {@code column(columnKey).get(rowKey)} still runs
 * quickly, since the row key is provided. However, {@code column(columnKey).size()} takes longer,
 * since an iteration across all row keys occurs.
 *
 * <p>Note that this implementation is not synchronized. If multiple threads access this table
 * concurrently and one of the threads modifies the table, it must be synchronized externally.
 *
 * @author Jared Levy
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
class StandardTable<R, C, V> extends AbstractTable<R, C, V> implements Serializable {
  @GwtTransient final Map<R, Map<C, V>> backingMap;
  @GwtTransient final Supplier<? extends Map<C, V>> factory;

  StandardTable(Map<R, Map<C, V>> backingMap, Supplier<? extends Map<C, V>> factory) {
    this.backingMap = backingMap;
    this.factory = factory;
  }

  // Accessors

  @Override
  public boolean contains(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
    return rowKey != null && columnKey != null && super.contains(rowKey, columnKey);
  }

  @Override
  public boolean containsColumn(@CheckForNull Object columnKey) {
    if (columnKey == null) {
      return false;
    }
    for (Map<C, V> map : backingMap.values()) {
      if (safeContainsKey(map, columnKey)) {
        return true;
      }
    }
    return false;
  }

  @Override
  public boolean containsRow(@CheckForNull Object rowKey) {
    return rowKey != null && safeContainsKey(backingMap, rowKey);
  }

  @Override
  public boolean containsValue(@CheckForNull Object value) {
    return value != null && super.containsValue(value);
  }

  @Override
  @CheckForNull
  public V get(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
    return (rowKey == null || columnKey == null) ? null : super.get(rowKey, columnKey);
  }

  @Override
  public boolean isEmpty() {
    return backingMap.isEmpty();
  }

  @Override
  public int size() {
    int size = 0;
    for (Map<C, V> map : backingMap.values()) {
      size += map.size();
    }
    return size;
  }

  // Mutators

  @Override
  public void clear() {
    backingMap.clear();
  }

  private Map<C, V> getOrCreate(R rowKey) {
    Map<C, V> map = backingMap.get(rowKey);
    if (map == null) {
      map = factory.get();
      backingMap.put(rowKey, map);
    }
    return map;
  }

  @CanIgnoreReturnValue
  @Override
  @CheckForNull
  public V put(R rowKey, C columnKey, V value) {
    checkNotNull(rowKey);
    checkNotNull(columnKey);
    checkNotNull(value);
    return getOrCreate(rowKey).put(columnKey, value);
  }

  @CanIgnoreReturnValue
  @Override
  @CheckForNull
  public V remove(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
    if ((rowKey == null) || (columnKey == null)) {
      return null;
    }
    Map<C, V> map = safeGet(backingMap, rowKey);
    if (map == null) {
      return null;
    }
    V value = map.remove(columnKey);
    if (map.isEmpty()) {
      backingMap.remove(rowKey);
    }
    return value;
  }

  @CanIgnoreReturnValue
  private Map<R, V> removeColumn(@CheckForNull Object column) {
    Map<R, V> output = new LinkedHashMap<>();
    Iterator<Entry<R, Map<C, V>>> iterator = backingMap.entrySet().iterator();
    while (iterator.hasNext()) {
      Entry<R, Map<C, V>> entry = iterator.next();
      V value = entry.getValue().remove(column);
      if (value != null) {
        output.put(entry.getKey(), value);
        if (entry.getValue().isEmpty()) {
          iterator.remove();
        }
      }
    }
    return output;
  }

  private boolean containsMapping(
      @CheckForNull Object rowKey, @CheckForNull Object columnKey, @CheckForNull Object value) {
    return value != null && value.equals(get(rowKey, columnKey));
  }

  /** Remove a row key / column key / value mapping, if present. */
  private boolean removeMapping(
      @CheckForNull Object rowKey, @CheckForNull Object columnKey, @CheckForNull Object value) {
    if (containsMapping(rowKey, columnKey, value)) {
      remove(rowKey, columnKey);
      return true;
    }
    return false;
  }

  // Views

  /**
   * Abstract set whose {@code isEmpty()} returns whether the table is empty and whose {@code
   * clear()} clears all table mappings.
   */
  @WeakOuter
  private abstract class TableSet<T> extends ImprovedAbstractSet<T> {
    @Override
    public boolean isEmpty() {
      return backingMap.isEmpty();
    }

    @Override
    public void clear() {
      backingMap.clear();
    }
  }

  /**
   * {@inheritDoc}
   *
   * <p>The set's iterator traverses the mappings for the first row, the mappings for the second
   * row, and so on.
   *
   * <p>Each cell is an immutable snapshot of a row key / column key / value mapping, taken at the
   * time the cell is returned by a method call to the set or its iterator.
   */
  @Override
  public Set<Cell<R, C, V>> cellSet() {
    return super.cellSet();
  }

  @Override
  Iterator<Cell<R, C, V>> cellIterator() {
    return new CellIterator();
  }

  private class CellIterator implements Iterator<Cell<R, C, V>> {
    final Iterator<Entry<R, Map<C, V>>> rowIterator = backingMap.entrySet().iterator();
    @CheckForNull Entry<R, Map<C, V>> rowEntry;
    Iterator<Entry<C, V>> columnIterator = Iterators.emptyModifiableIterator();

    @Override
    public boolean hasNext() {
      return rowIterator.hasNext() || columnIterator.hasNext();
    }

    @Override
    public Cell<R, C, V> next() {
      if (!columnIterator.hasNext()) {
        rowEntry = rowIterator.next();
        columnIterator = rowEntry.getValue().entrySet().iterator();
      }
      /*
       * requireNonNull is safe because:
       *
       * - columnIterator started off pointing to an empty iterator, so we must have entered the
       *   `if` body above at least once. Thus, if we got this far, that `if` body initialized
       *   rowEntry at least once.
       *
       * - The only case in which rowEntry is cleared (during remove() below) happens only if the
       *   caller removed every element from columnIterator. During that process, we would have had
       *   to iterate it to exhaustion. Then we can apply the logic above about an empty
       *   columnIterator. (This assumes no concurrent modification, but behavior under concurrent
       *   modification is undefined, anyway.)
       */
      requireNonNull(rowEntry);
      Entry<C, V> columnEntry = columnIterator.next();
      return Tables.immutableCell(rowEntry.getKey(), columnEntry.getKey(), columnEntry.getValue());
    }

    @Override
    public void remove() {
      columnIterator.remove();
      /*
       * requireNonNull is safe because:
       *
       * - columnIterator.remove() succeeded, so it must have returned a value, so it must have been
       * initialized by next() -- which initializes rowEntry, too.
       *
       * - rowEntry isn't cleared except below. If it was cleared below, then either
       *   columnIterator.remove() would have failed above (if the user hasn't called next() since
       *   then) or rowEntry would have been initialized by next() (as discussed above).
       */
      if (requireNonNull(rowEntry).getValue().isEmpty()) {
        rowIterator.remove();
        rowEntry = null;
      }
    }
  }

  @Override
  Spliterator<Cell<R, C, V>> cellSpliterator() {
    return CollectSpliterators.flatMap(
        backingMap.entrySet().spliterator(),
        (Entry<R, Map<C, V>> rowEntry) ->
            CollectSpliterators.map(
                rowEntry.getValue().entrySet().spliterator(),
                (Entry<C, V> columnEntry) ->
                    Tables.immutableCell(
                        rowEntry.getKey(), columnEntry.getKey(), columnEntry.getValue())),
        Spliterator.DISTINCT | Spliterator.SIZED,
        size());
  }

  @Override
  public Map<C, V> row(R rowKey) {
    return new Row(rowKey);
  }

  class Row extends IteratorBasedAbstractMap<C, V> {
    final R rowKey;

    Row(R rowKey) {
      this.rowKey = checkNotNull(rowKey);
    }

    @CheckForNull Map<C, V> backingRowMap;

    final void updateBackingRowMapField() {
      if (backingRowMap == null || (backingRowMap.isEmpty() && backingMap.containsKey(rowKey))) {
        backingRowMap = computeBackingRowMap();
      }
    }

    @CheckForNull
    Map<C, V> computeBackingRowMap() {
      return backingMap.get(rowKey);
    }

    // Call this every time we perform a removal.
    void maintainEmptyInvariant() {
      updateBackingRowMapField();
      if (backingRowMap != null && backingRowMap.isEmpty()) {
        backingMap.remove(rowKey);
        backingRowMap = null;
      }
    }

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      updateBackingRowMapField();
      return (key != null && backingRowMap != null) && Maps.safeContainsKey(backingRowMap, key);
    }

    @Override
    @CheckForNull
    public V get(@CheckForNull Object key) {
      updateBackingRowMapField();
      return (key != null && backingRowMap != null) ? Maps.safeGet(backingRowMap, key) : null;
    }

    @Override
    @CheckForNull
    public V put(C key, V value) {
      checkNotNull(key);
      checkNotNull(value);
      if (backingRowMap != null && !backingRowMap.isEmpty()) {
        return backingRowMap.put(key, value);
      }
      return StandardTable.this.put(rowKey, key, value);
    }

    @Override
    @CheckForNull
    public V remove(@CheckForNull Object key) {
      updateBackingRowMapField();
      if (backingRowMap == null) {
        return null;
      }
      V result = Maps.safeRemove(backingRowMap, key);
      maintainEmptyInvariant();
      return result;
    }

    @Override
    public void clear() {
      updateBackingRowMapField();
      if (backingRowMap != null) {
        backingRowMap.clear();
      }
      maintainEmptyInvariant();
    }

    @Override
    public int size() {
      updateBackingRowMapField();
      return (backingRowMap == null) ? 0 : backingRowMap.size();
    }

    @Override
    Iterator<Entry<C, V>> entryIterator() {
      updateBackingRowMapField();
      if (backingRowMap == null) {
        return Iterators.emptyModifiableIterator();
      }
      final Iterator<Entry<C, V>> iterator = backingRowMap.entrySet().iterator();
      return new Iterator<Entry<C, V>>() {
        @Override
        public boolean hasNext() {
          return iterator.hasNext();
        }

        @Override
        public Entry<C, V> next() {
          return wrapEntry(iterator.next());
        }

        @Override
        public void remove() {
          iterator.remove();
          maintainEmptyInvariant();
        }
      };
    }

    @Override
    Spliterator<Entry<C, V>> entrySpliterator() {
      updateBackingRowMapField();
      if (backingRowMap == null) {
        return Spliterators.emptySpliterator();
      }
      return CollectSpliterators.map(backingRowMap.entrySet().spliterator(), this::wrapEntry);
    }

    Entry<C, V> wrapEntry(final Entry<C, V> entry) {
      return new ForwardingMapEntry<C, V>() {
        @Override
        protected Entry<C, V> delegate() {
          return entry;
        }

        @Override
        public V setValue(V value) {
          return super.setValue(checkNotNull(value));
        }

        @Override
        public boolean equals(@CheckForNull Object object) {
          // TODO(lowasser): identify why this affects GWT tests
          return standardEquals(object);
        }
      };
    }
  }

  /**
   * {@inheritDoc}
   *
   * <p>The returned map's views have iterators that don't support {@code remove()}.
   */
  @Override
  public Map<R, V> column(C columnKey) {
    return new Column(columnKey);
  }

  private class Column extends ViewCachingAbstractMap<R, V> {
    final C columnKey;

    Column(C columnKey) {
      this.columnKey = checkNotNull(columnKey);
    }

    @Override
    @CheckForNull
    public V put(R key, V value) {
      return StandardTable.this.put(key, columnKey, value);
    }

    @Override
    @CheckForNull
    public V get(@CheckForNull Object key) {
      return StandardTable.this.get(key, columnKey);
    }

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return StandardTable.this.contains(key, columnKey);
    }

    @Override
    @CheckForNull
    public V remove(@CheckForNull Object key) {
      return StandardTable.this.remove(key, columnKey);
    }

    /** Removes all {@code Column} mappings whose row key and value satisfy the given predicate. */
    @CanIgnoreReturnValue
    boolean removeFromColumnIf(Predicate<? super Entry<R, V>> predicate) {
      boolean changed = false;
      Iterator<Entry<R, Map<C, V>>> iterator = backingMap.entrySet().iterator();
      while (iterator.hasNext()) {
        Entry<R, Map<C, V>> entry = iterator.next();
        Map<C, V> map = entry.getValue();
        V value = map.get(columnKey);
        if (value != null && predicate.apply(Maps.immutableEntry(entry.getKey(), value))) {
          map.remove(columnKey);
          changed = true;
          if (map.isEmpty()) {
            iterator.remove();
          }
        }
      }
      return changed;
    }

    @Override
    Set<Entry<R, V>> createEntrySet() {
      return new EntrySet();
    }

    @WeakOuter
    private class EntrySet extends ImprovedAbstractSet<Entry<R, V>> {
      @Override
      public Iterator<Entry<R, V>> iterator() {
        return new EntrySetIterator();
      }

      @Override
      public int size() {
        int size = 0;
        for (Map<C, V> map : backingMap.values()) {
          if (map.containsKey(columnKey)) {
            size++;
          }
        }
        return size;
      }

      @Override
      public boolean isEmpty() {
        return !containsColumn(columnKey);
      }

      @Override
      public void clear() {
        removeFromColumnIf(alwaysTrue());
      }

      @Override
      public boolean contains(@CheckForNull Object o) {
        if (o instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) o;
          return containsMapping(entry.getKey(), columnKey, entry.getValue());
        }
        return false;
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        if (obj instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) obj;
          return removeMapping(entry.getKey(), columnKey, entry.getValue());
        }
        return false;
      }

      @Override
      public boolean retainAll(Collection<?> c) {
        return removeFromColumnIf(not(in(c)));
      }
    }

    private class EntrySetIterator extends AbstractIterator<Entry<R, V>> {
      final Iterator<Entry<R, Map<C, V>>> iterator = backingMap.entrySet().iterator();

      @Override
      @CheckForNull
      protected Entry<R, V> computeNext() {
        while (iterator.hasNext()) {
          final Entry<R, Map<C, V>> entry = iterator.next();
          if (entry.getValue().containsKey(columnKey)) {
            @WeakOuter
            class EntryImpl extends AbstractMapEntry<R, V> {
              @Override
              public R getKey() {
                return entry.getKey();
              }

              @Override
              public V getValue() {
                return entry.getValue().get(columnKey);
              }

              @Override
              public V setValue(V value) {
                /*
                 * The cast is safe because of the containsKey check above. (Well, it's possible for
                 * the map to change between that call and this one. But if that happens, the
                 * behavior is undefined because of the concurrent mutation.)
                 *
                 * (Our prototype checker happens to be "smart" enough to understand this for the
                 * *get* call in getValue but not for the *put* call here.)
                 *
                 * (Arguably we should use requireNonNull rather than uncheckedCastNullableTToT: We
                 * know that V is a non-null type because that's the only kind of value type that
                 * StandardTable supports. Thus, requireNonNull is safe as long as the cell is still
                 * present. (And if it's not present, behavior is undefined.) However, that's a
                 * behavior change relative to the old code, so it didn't seem worth risking.)
                 */
                return uncheckedCastNullableTToT(
                    entry.getValue().put(columnKey, checkNotNull(value)));
              }
            }
            return new EntryImpl();
          }
        }
        return endOfData();
      }
    }

    @Override
    Set<R> createKeySet() {
      return new KeySet();
    }

    @WeakOuter
    private class KeySet extends Maps.KeySet<R, V> {
      KeySet() {
        super(Column.this);
      }

      @Override
      public boolean contains(@CheckForNull Object obj) {
        return StandardTable.this.contains(obj, columnKey);
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        return StandardTable.this.remove(obj, columnKey) != null;
      }

      @Override
      public boolean retainAll(final Collection<?> c) {
        return removeFromColumnIf(Maps.<R>keyPredicateOnEntries(not(in(c))));
      }
    }

    @Override
    Collection<V> createValues() {
      return new Values();
    }

    @WeakOuter
    private class Values extends Maps.Values<R, V> {
      Values() {
        super(Column.this);
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        return obj != null && removeFromColumnIf(Maps.<V>valuePredicateOnEntries(equalTo(obj)));
      }

      @Override
      public boolean removeAll(final Collection<?> c) {
        return removeFromColumnIf(Maps.<V>valuePredicateOnEntries(in(c)));
      }

      @Override
      public boolean retainAll(final Collection<?> c) {
        return removeFromColumnIf(Maps.<V>valuePredicateOnEntries(not(in(c))));
      }
    }
  }

  @Override
  public Set<R> rowKeySet() {
    return rowMap().keySet();
  }

  @LazyInit @CheckForNull private transient Set<C> columnKeySet;

  /**
   * {@inheritDoc}
   *
   * <p>The returned set has an iterator that does not support {@code remove()}.
   *
   * <p>The set's iterator traverses the columns of the first row, the columns of the second row,
   * etc., skipping any columns that have appeared previously.
   */
  @Override
  public Set<C> columnKeySet() {
    Set<C> result = columnKeySet;
    return (result == null) ? columnKeySet = new ColumnKeySet() : result;
  }

  @WeakOuter
  private class ColumnKeySet extends TableSet<C> {
    @Override
    public Iterator<C> iterator() {
      return createColumnKeyIterator();
    }

    @Override
    public int size() {
      return Iterators.size(iterator());
    }

    @Override
    public boolean remove(@CheckForNull Object obj) {
      if (obj == null) {
        return false;
      }
      boolean changed = false;
      Iterator<Map<C, V>> iterator = backingMap.values().iterator();
      while (iterator.hasNext()) {
        Map<C, V> map = iterator.next();
        if (map.keySet().remove(obj)) {
          changed = true;
          if (map.isEmpty()) {
            iterator.remove();
          }
        }
      }
      return changed;
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      checkNotNull(c);
      boolean changed = false;
      Iterator<Map<C, V>> iterator = backingMap.values().iterator();
      while (iterator.hasNext()) {
        Map<C, V> map = iterator.next();
        // map.keySet().removeAll(c) can throw a NPE when map is a TreeMap with
        // natural ordering and c contains a null.
        if (Iterators.removeAll(map.keySet().iterator(), c)) {
          changed = true;
          if (map.isEmpty()) {
            iterator.remove();
          }
        }
      }
      return changed;
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      checkNotNull(c);
      boolean changed = false;
      Iterator<Map<C, V>> iterator = backingMap.values().iterator();
      while (iterator.hasNext()) {
        Map<C, V> map = iterator.next();
        if (map.keySet().retainAll(c)) {
          changed = true;
          if (map.isEmpty()) {
            iterator.remove();
          }
        }
      }
      return changed;
    }

    @Override
    public boolean contains(@CheckForNull Object obj) {
      return containsColumn(obj);
    }
  }

  /** Creates an iterator that returns each column value with duplicates omitted. */
  Iterator<C> createColumnKeyIterator() {
    return new ColumnKeyIterator();
  }

  private class ColumnKeyIterator extends AbstractIterator<C> {
    // Use the same map type to support TreeMaps with comparators that aren't
    // consistent with equals().
    final Map<C, V> seen = factory.get();
    final Iterator<Map<C, V>> mapIterator = backingMap.values().iterator();
    Iterator<Entry<C, V>> entryIterator = Iterators.emptyIterator();

    @Override
    @CheckForNull
    protected C computeNext() {
      while (true) {
        if (entryIterator.hasNext()) {
          Entry<C, V> entry = entryIterator.next();
          if (!seen.containsKey(entry.getKey())) {
            seen.put(entry.getKey(), entry.getValue());
            return entry.getKey();
          }
        } else if (mapIterator.hasNext()) {
          entryIterator = mapIterator.next().entrySet().iterator();
        } else {
          return endOfData();
        }
      }
    }
  }

  /**
   * {@inheritDoc}
   *
   * <p>The collection's iterator traverses the values for the first row, the values for the second
   * row, and so on.
   */
  @Override
  public Collection<V> values() {
    return super.values();
  }

  @LazyInit @CheckForNull private transient Map<R, Map<C, V>> rowMap;

  @Override
  public Map<R, Map<C, V>> rowMap() {
    Map<R, Map<C, V>> result = rowMap;
    return (result == null) ? rowMap = createRowMap() : result;
  }

  Map<R, Map<C, V>> createRowMap() {
    return new RowMap();
  }

  @WeakOuter
  class RowMap extends ViewCachingAbstractMap<R, Map<C, V>> {
    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return containsRow(key);
    }

    // performing cast only when key is in backing map and has the correct type
    @SuppressWarnings("unchecked")
    @Override
    @CheckForNull
    public Map<C, V> get(@CheckForNull Object key) {
      // requireNonNull is safe because of the containsRow check.
      return containsRow(key) ? row((R) requireNonNull(key)) : null;
    }

    @Override
    @CheckForNull
    public Map<C, V> remove(@CheckForNull Object key) {
      return (key == null) ? null : backingMap.remove(key);
    }

    @Override
    protected Set<Entry<R, Map<C, V>>> createEntrySet() {
      return new EntrySet();
    }

    @WeakOuter
    private final class EntrySet extends TableSet<Entry<R, Map<C, V>>> {
      @Override
      public Iterator<Entry<R, Map<C, V>>> iterator() {
        return Maps.asMapEntryIterator(
            backingMap.keySet(),
            new Function<R, Map<C, V>>() {
              @Override
              public Map<C, V> apply(R rowKey) {
                return row(rowKey);
              }
            });
      }

      @Override
      public int size() {
        return backingMap.size();
      }

      @Override
      public boolean contains(@CheckForNull Object obj) {
        if (obj instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) obj;
          return entry.getKey() != null
              && entry.getValue() instanceof Map
              && Collections2.safeContains(backingMap.entrySet(), entry);
        }
        return false;
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        if (obj instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) obj;
          return entry.getKey() != null
              && entry.getValue() instanceof Map
              && backingMap.entrySet().remove(entry);
        }
        return false;
      }
    }
  }

  @LazyInit @CheckForNull private transient ColumnMap columnMap;

  @Override
  public Map<C, Map<R, V>> columnMap() {
    ColumnMap result = columnMap;
    return (result == null) ? columnMap = new ColumnMap() : result;
  }

  @WeakOuter
  private class ColumnMap extends ViewCachingAbstractMap<C, Map<R, V>> {
    // The cast to C occurs only when the key is in the map, implying that it
    // has the correct type.
    @SuppressWarnings("unchecked")
    @Override
    @CheckForNull
    public Map<R, V> get(@CheckForNull Object key) {
      // requireNonNull is safe because of the containsColumn check.
      return containsColumn(key) ? column((C) requireNonNull(key)) : null;
    }

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return containsColumn(key);
    }

    @Override
    @CheckForNull
    public Map<R, V> remove(@CheckForNull Object key) {
      return containsColumn(key) ? removeColumn(key) : null;
    }

    @Override
    public Set<Entry<C, Map<R, V>>> createEntrySet() {
      return new ColumnMapEntrySet();
    }

    @Override
    public Set<C> keySet() {
      return columnKeySet();
    }

    @Override
    Collection<Map<R, V>> createValues() {
      return new ColumnMapValues();
    }

    @WeakOuter
    private final class ColumnMapEntrySet extends TableSet<Entry<C, Map<R, V>>> {
      @Override
      public Iterator<Entry<C, Map<R, V>>> iterator() {
        return Maps.asMapEntryIterator(
            columnKeySet(),
            new Function<C, Map<R, V>>() {
              @Override
              public Map<R, V> apply(C columnKey) {
                return column(columnKey);
              }
            });
      }

      @Override
      public int size() {
        return columnKeySet().size();
      }

      @Override
      public boolean contains(@CheckForNull Object obj) {
        if (obj instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) obj;
          if (containsColumn(entry.getKey())) {
            // requireNonNull is safe because of the containsColumn check.
            return requireNonNull(get(entry.getKey())).equals(entry.getValue());
          }
        }
        return false;
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        /*
         * `o instanceof Entry` is guaranteed by `contains`, but we check it here to satisfy our
         * nullness checker.
         */
        if (contains(obj) && obj instanceof Entry) {
          Entry<?, ?> entry = (Entry<?, ?>) obj;
          removeColumn(entry.getKey());
          return true;
        }
        return false;
      }

      @Override
      public boolean removeAll(Collection<?> c) {
        /*
         * We can't inherit the normal implementation (which calls
         * Sets.removeAllImpl(Set, *Collection*)) because, under some
         * circumstances, it attempts to call columnKeySet().iterator().remove,
         * which is unsupported.
         */
        checkNotNull(c);
        return Sets.removeAllImpl(this, c.iterator());
      }

      @Override
      public boolean retainAll(Collection<?> c) {
        checkNotNull(c);
        boolean changed = false;
        for (C columnKey : Lists.newArrayList(columnKeySet().iterator())) {
          if (!c.contains(Maps.immutableEntry(columnKey, column(columnKey)))) {
            removeColumn(columnKey);
            changed = true;
          }
        }
        return changed;
      }
    }

    @WeakOuter
    private class ColumnMapValues extends Maps.Values<C, Map<R, V>> {
      ColumnMapValues() {
        super(ColumnMap.this);
      }

      @Override
      public boolean remove(@CheckForNull Object obj) {
        for (Entry<C, Map<R, V>> entry : ColumnMap.this.entrySet()) {
          if (entry.getValue().equals(obj)) {
            removeColumn(entry.getKey());
            return true;
          }
        }
        return false;
      }

      @Override
      public boolean removeAll(Collection<?> c) {
        checkNotNull(c);
        boolean changed = false;
        for (C columnKey : Lists.newArrayList(columnKeySet().iterator())) {
          if (c.contains(column(columnKey))) {
            removeColumn(columnKey);
            changed = true;
          }
        }
        return changed;
      }

      @Override
      public boolean retainAll(Collection<?> c) {
        checkNotNull(c);
        boolean changed = false;
        for (C columnKey : Lists.newArrayList(columnKeySet().iterator())) {
          if (!c.contains(column(columnKey))) {
            removeColumn(columnKey);
            changed = true;
          }
        }
        return changed;
      }
    }
  }

  private static final long serialVersionUID = 0;
}